Exhaust gas emitted from an internal combustion engine, particularly a diesel engine, is a heterogeneous mixture that contains gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and oxides of nitrogen (“NOx”) as well as condensed phase materials (liquids and solids) that constitute particulate matter. Catalyst compositions typically disposed on catalyst supports or substrates are provided in an engine exhaust system to convert certain, or all of these exhaust constituents into non-regulated exhaust gas components.
Sensors are provided in the engine exhaust system to measure the levels of the emissions. In particular, a NOx sensor can be disposed within the engine exhaust system to measure the NOx levels in the exhaust gas. The internal combustion engine is controlled based on the NOx sensor readings to reduce the level of NOx. Thus, proper operation of the engine is based on proper operation of the NOx sensor.
Accordingly, it is desirable to provide systems and methods for monitoring the functionality of the NOx sensors to ensure proper operation.